This invention relates to a novel method for controlling a manufacturing process utilizing control charts.
Control charts are a well-known and important means for monitoring the state of a manufacturing process. An example of typical control charts is shown in FIGS. 1A, 1B (numerals 10,12).
In particular, FIG. 1A shows an {overscore (x)} control chart 10 comprising an ordinate time series of a manufacturing variable (sample number) and an abscissa control limit window (upper control limit UCL and lower control limit LCL). The FIG. 1A control chart 10 provides data in response to an inquiry: Is the manufacturing process centered?
FIG. 1B shows an R-control chart 12 comprising an ordinate time series of a manufacturing variable (sample number) and an abscissa control limit window, and is complimentary to the FIG. 1A control chart 10 in that the control chart 12 provides data in response to the variability of the manufacturing process.
Our work centers on a critique of the capabilities of extant control charts, of the type illustratively shown in FIG. 1, to an end of disclosing novel methodology which can qualitatively extend and exploit their utility.
To this end, we note that present control charts have inherent problems or deficiencies, including:
1) A fraction of good manufactured products are incorrectly rejected due to false alarms which are inherent in the statistical nature of present control charts.
2) A fraction of unacceptable (bad) products are incorrectly accepted due to false alarms, etc.
3) Decisions for acceptance/rejection are binary, and therefore cannot incorporate an operatives prior knowledge and domain expertise or desired credence level.
As just alluded to, we have now discovered a novel method that can preserve the advantages and features of extant control charts, while it extends and more fully exploits their utility and potential capability.
The novel method comprises the steps of:
(1) constructing a control chart comprising a time series of a manufacturing variable of interest operated upon by process control limits;
(2) computing specified confidence intervals for the control limits; and
(3) generating a signal based on a comparison of a time series measurement of the manufacturing variable of interest with the specified confidence intervals computed for the control limits.
The invention, as defined, can realize significant advantages. For example, and with reference to problems and deficiencies of extant control charts of the type noted abovexe2x80x94it can minimize the fraction of good manufacturing product falsely rejected; it can minimize the fraction of bad product which would otherwise be incorrectly accepted; and, it can enable users to utilize prior knowledge i.e., binary decisions are enriched. Other advantages are disclosed below.